1. Field of the Invention
The present invention relates to a technique of converting colorimetric values to display signals input into a color display device and a technique of inversely converting the display signals to the colorimetric values.
2. Description of the Related Art
A conventional technique reads an original color image, for example, from a color film, color photographic paper, or a color print, with a color scanner, obtains three color signals of R (red), G (green), and B (blue) corresponding to the input original color image, and gives the obtained RGB color signals to a color monitor to display a color image in an interactive manner for the purpose of checking the input color image or carrying out the required processing.
The RGB color signals read with the color scanner depend upon only the characteristics of a color separation filter of the color scanner. Transmission of the RGB color signals to the color monitor without any processing accordingly does not give a color image that is sufficiently close to the original color image.
In the field of image processing for prints and newspapers, four color inks C (cyan), M (magenta), Y (yellow), and K(black) are used for color printing. Each pixel of an original image for color printing is subjected to color conversion and consists of four color signals of C, M, Y, and K. In the intermediate stage of image processing, in order to preview an image being processed or an expected final image, the CMYK color signals for color printing are converted into the RGB color signals and given to the color monitor for display of a color image.
Simple conversion of the CMYK color signals for color printing to the RGB color signals and transmission of the converted RGB color signals to the color monitor, however, does not give a color image that is sufficiently close to an actual color printed image.
The RGB color signals depending upon only the characteristics of the color separation filter or the CMYK color signals for color printing should be subjected to color conversion to RGB color signals for a target color monitor. Such conversion process enables a desired color image to be displayed on the color monitor.
The color monitor can display only the colors included in a color range defined by fluorescent materials of three colors, red, green, and blue (that is, in the gamut intrinsic to the color monitor), whereas the gamut of an original color image or the gamut of a color printed image often includes colors that are located out of the color range defined by the fluorescent materials of three colors (red, green, and blue) of the color monitor.
When the values of the RGB color signals for the color monitor obtained by the color conversion are normalized in the range of 0 to 1, at least one signal out of the RGB color signals for the color monitor has a value of less than 0 (that is, a negative value) or a value of greater than 1 (that is, a value exceeding 100%), with respect to the colors located out of the color range defined by the fluorescent materials of three colors (red, green, and blue) of the color monitor.
Normalization of the values of the RGB color signals for the color monitor in the range of 0 to 1 implies that the values of the RGB color signals required for expression of white on the color monitor are respectively set equal to 1 (that is, the maximum value) and that the values of the RGB color signals required for expression of black on the color monitor are respectively set equal to 0 (that is, the minimum value). The fluorescent material can not accordingly emit at the values of greater than 1 or at the values of less than 0.
In such cases, the conventional technique limits the values of the RGB color signals for the color monitor to the maximum value xe2x80x981xe2x80x99 or the minimum value xe2x80x980xe2x80x99 even when the values of greater than 1 or the values of less than 0 are required as the values of the RGB color signals for the color monitor.
In case that a desired color included in an original color image or a color printed image is out of the color range reproducible by the fluorescent materials of the color monitor, the values of the RGB color signals for the color monitor are uniformly limited to the maximum value xe2x80x981xe2x80x99 or the minimum value xe2x80x980xe2x80x99 with respect to the desired color (for example, the color of high saturation). This arises a significant problem of abnormal tone in an area of the desired color.
In case that a desired color included in an original color image or a color printed image exists over the border of the inside and the outside of the color range reproducible by the fluorescent materials of the color monitor, all the values of the three color signals RGB are not uniformly limited to the maximum value xe2x80x981xe2x80x99 or the minimum value xe2x80x980xe2x80x99 on the border of the color range. Only the value of the color signal that is greater than the maximum xe2x80x981xe2x80x99 or is less than the minimum xe2x80x980xe2x80x99 is limited to the maximum value xe2x80x981xe2x80x99 or the minimum value xe2x80x980xe2x80x99, while the values of the other color signals continue variation. This causes the hue to abruptly change in the vicinity of the border of the color range.
The area of colors of high saturation is generally prominent in an image. The abnormal tone or the change in hue in such prominent areas often lead to a critical problem.
The object of the present invention is thus to provide a method of and an apparatus for color conversion, which effectively prevents abnormal tone or a change in hue in an area of a desired color that exists out of a reproducible color range.
At least part of the above and the other related objects is realized by a first method of color conversion, which converts colorimetric values of a composite color into mixing quantities of device primary colors in additive color mixture according to a first conversion relation and subsequently converts the mixing quantities of the device primary colors into intensities of display signals according to a second conversion relation, respectively. A relievable area that is out of and adjacent to a reproducible range and within a predetermined maximum relievable range is determined for at least one selected device primary color among the device primary colors as a function of the mixing quantities of the remaining device primary colors other than the selected device primary. When the mixing quantity of the selected device primary color is within the reproducible range, the mixing quantity of the selected device primary color is converted into the intensity of the display signal within the first conversion range preset in a definition range. When the mixing quantity of the selected device primary color is within the relievable range, on the other hand, the mixing quantity of the selected device primary color is converted into the intensity of the display signal within the second conversion range other than the first conversion range in the definition range.
The present invention is also directed to a first color conversion apparatus which converts colorimetric values representing a composite color into display signals that are to be input into a target color display device to reproduce the composite color, the display signals being related to mixing quantities of device primary colors in additive color mixture, the device primary colors being single-color components reproduced by respective single-color elements of the target color display device, the composite color being reproduced by the target color display device as a mixture of the device primary colors mixed by respective mixing quantities responsive to the display signals, the target color display device having a reproducible range of the mixing quantity for each of the device primary colors.
The first color conversion apparatus includes: means for defining a definition range of intensities of the display signals, wherein those display signals whose intensities are within the definition range are to be converted in the target color display device to the mixture quantities within the reproducible range, the definition range being divided into a first conversion range and a second conversion range; a first conversion unit which converts the colorimetric values of the composite color into the mixing quantities of the device primary colors according to a first conversion relation; and a second conversion unit which converts the mixing quantities of the device primary colors into the intensities of the display signals according to a second conversion relation, respectively. The second conversion unit determines a relievable range for at least one selected device primary color among the device primary colors as a function of the mixing quantities of the remaining device primary colors other than the selected device primary color, the relievable range being out of and adjacent to the reproducible range and within a predetermined maximum relievable range. When the mixing quantity of the selected device primary color is within the reproducible range, the second conversion unit converts the mixing quantity of the selected device primary color into the intensity of the display signal within the first conversion range. When the mixing quantity of the selected device primary color is within the relievable range, the second conversion unit converts the mixing quantity of the selected device primary color into the intensity of the display signal within the second conversion range.
With respect to a desired color that is out of the predetermined range reproducible by a color display device, when the mixing quantity of a selected device primary color among the mixing quantities of the device primary colors in additive color mixture, which are obtained by converting the colorimetric values of the desired color, is within the relievable range, the mixing quantity is converted into the intensity of the display signal that is within the second conversion range. This technique does not lose the information regarding the desired color, but keeps the information of the selected device primary color as the intensity of the display signal. This effectively prevents abnormal tone or a change in hue in the area of the desired color.
In accordance with one preferable application of the first method or the first color conversion apparatus, the first conversion relation represents conversion from the colorimetric values to mixing quantities of virtual device primary colors of a virtual color display device, the virtual color display device having virtual single-color elements reproducing virtual single-color components associated with the single-color components reproduced by the single-color elements of the target color display device, respectively, wherein at least one selected virtual single-color component is determined so that the selected virtual single-color component has an identical hue with that of the associated single-color component and a higher saturation than that of the associated single-color component.
Compared with the target color display device, the virtual color display device has an extended reproducible color range in the direction of higher saturation. Namely the virtual color display device can reproduce some colors that are not reproducible by the target color display device (more concretely, the colors of high saturation that are immediately out of the color range reproducible by the target color display device). The first method and the first color conversion apparatus of the present invention convert the colorimetric values into the mixing quantities of the device primary colors in additive color mixture, according to the converting relation specified for the virtual color display device. This technique maintains normal tone and interferes with a change in hue in the area of a desired color that is out of the color range reproducible by the target color display device but within the color range reproducible by the virtual color display device.
Part of the objects is also realized by a second method of color conversion, which converts intensities of display signals, which respectively correspond to device primary colors in additive color mixture, into mixing quantities of the device primary colors according to a first converting relation, respectively, and subsequently converts the mixing quantities of the device primary colors into colorimetric values according to a second converting relation. A relievable range that is out of and adjacent to a reproducible range and within a predetermined maximum relievable range is determined for at least one selected device primary color among the device primary colors as a function of the intensities of the display signals respectively correspond to the remaining device primary colors other than the selected device primary color. When the intensity of the display signal corresponding to the selected device primary color is within a first conversion range preset in a definition range, the intensity of the display signal is converted into the mixing quantity of the selected device primary color within the reproducible range. When the intensity of the display signal corresponding to the selected device primary color is within a second conversion range other than the first conversion range in the definition range, the intensity of the display signal is converted into the mixing quantity of the selected device primary color within the relievable range.
The second method of color conversion enables display signals to be input into a color display device into colorimetric values.
In accordance with one preferable application of the second method, the second conversion relation represents conversion from mixing quantities of virtual device primary colors of a virtual color display device to the colorimetric values, the virtual color display device having virtual single-color elements reproducing virtual single-color components associated with the single-color components reproduced by the single-color elements of the target color display device, respectively, wherein at least one selected virtual single-color component is determined so that the selected virtual single-color component has an identical hue with that of the associated single-color component and a higher saturation than that of the associated single-color component.
The present invention is further directed to a second color conversion apparatus which carries out color conversion. The second color conversion apparatus includes: a first color conversion device which converts colorimetric values representing a composite color into display signals that are to be input into a target color display device to reproduce the composite color, the display signals being related to mixing quantities of device primary colors in additive color mixture, the device primary colors being single-color components reproduced by respective single-color elements of the target color display device, the composite color being reproduced by the target color display device as a mixture of the device primary colors mixed by respective mixing quantities responsive to the display signals, wherein those display signals whose intensities are within a definition range are to be converted in the target color display device to the mixture quantities within a reproducible range, the definition range being divided into a first conversion range and a second conversion range; and a second color conversion device which comprises a color conversion look-up table including representative points for color conversion and converts the intensities of the display signals obtained by said first color conversion device into values of a predetermined color system using the representative points included in said color conversion look-up table and interpolation with the representative points.
The first color conversion device includes: a first conversion unit which converts the colorimetric values of the composite color into the mixing quantities of the device primary colors according to a first conversion relation; and a second conversion unit which converts the mixing quantities of the device primary colors into the intensities of the display signals according to a second conversion relation, respectively.
The second conversion unit determines a relievable range for at least one selected device primary color among the device primary colors as a function of the mixing quantities of the remaining device primary colors other than the selected device primary color, the relievable range being out of and adjacent to the reproducible range and within a predetermined maximum relievable range. When the mixing quantity of the selected device primary color is within the reproducible range, the second conversion unit converting the mixing quantity of the selected device primary color into the intensity of the display signal within the first conversion range. When the mixing quantity of the selected device primary color is within the relievable range, the second conversion unit converting the mixing quantity of the selected device primary color into the intensity of the display signal within the second conversion range.
An address of the color conversion look-up table used in the second color conversion device is given by combinations of the intensities of the display signals respectively corresponding to the device primary colors, the combinations of the intensities of the display signals comprising at least specific combinations including a specific boundary value as the intensity of the display signal corresponding to the selected device primary color, the specific boundary value being on a boundary between the first conversion range and the second conversion range.
This technique effectively prevents a specific area over the first conversion range and the second conversion range from being interpolated with an identical coefficient, thereby giving appropriate values in the color specification system.
In accordance with one preferable application of the second color conversion apparatus, the first conversion relation used by the first conversion unit represents conversion from the colorimetric values to mixing quantities of virtual device primary colors of a virtual color display device, the virtual color display device having virtual single-color elements reproducing virtual single-color components associated with the single-color components reproduced by the single-color elements of the target color display device, respectively, wherein at least one selected virtual single-color component is determined so that the selected virtual single-color component has an identical hue with that of the associated single-color component and a higher saturation than that of the associated single-color component.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.